Our goal is to understand how T cell functions and its change with aging. We focus on 1) how memory T cells are generated, maintained, and functioned in an immune response, and 2) identification of age-associated changes in chromatin and transcriptome of CD8 T cells. We are particularly interested in investigating the mechanism of differential transcription at levels of chromatin state. We apply ChIP-Seq, ATAC-seq, microarray, RNA-seq and scRNAseq to identify genes associated subset populations that differentially expressed in nave and memory T cells and to quantitate the chromatin states (measured by ChIP-seq and ATAC-seq) in the corresponding gene loci, and their changes with age. The parallel comparative analyses of gene expression and chromatin states reveal the chromatin basis of the differential gene expression and the role of chromatin in memory T cell functions and in CD8 T cell aging. Transition from resting to cell cycle in response to antigenic stimulation is an essential step for nave CD8+ T cells to differentiate to effector and memory cells. Leaving the resting state requires dramatic changes of chromatin status in the key cell cycle inhibitors but the details of these concerted events are not fully elucidated. Here, we showed that Ezh2, an enzymatic component of polycomb repressive complex 2 (PRC2) catalyzing the trimethylation of lysine 27 on histone 3 (H3K27me3), regulates activation induced nave CD8+ T cells proliferation and apoptosis. Upon deletion of Ezh2 during thymocyte development (Ezh2fl/flCd4Cre+ mice), naive CD8+ T cells displayed impaired proliferation and increased apoptosis in response to antigen stimulation. However, naive CD8+ T cells only had impaired proliferation but no increase in apoptosis when Ezh2 was deleted after activation (Ezh2fl/flGzmBCre+ mice), suggesting cell cycle and apoptosis are temporally separable events controlled by Ezh2. We then showed that deletion of Ezh2 resulted in the increase in expression of cyclin-dependent kinase inhibitors Cdkn2a (p16 and Arf) and Cdkn1c (p57) in activated nave CD8+ T cells as the consequence of reduced levels of H3K27me3 at these two gene loci. Finally, with real time imaging, we observed prolonged cell division times of nave CD8+ T cells in the absence of Ezh2 post in vitro stimulation. Together, these findings reveal that repression of Cdkn1c and Cdkn2a by Ezh2 plays a critical role in execution of activation-induced CD8+ T cell proliferation. Lysine specific methyl-transferase 2D (Kmt2d) catalyzes the mono-methylation of histone 3 lysine 4 (H3K4me1) and play a critical role in regulatory T cell generation via modulating Foxp3 gene expression. Here we report a role of Kmt2d in nave CD8+ T cell survival upon activation. First, we found that the number of CD8+ T cells, in particular nave CD8+ T cells (CD62Lhi/CD44lo), in spleen was greatly decreased in the absence of Kmt2d (Kmt2dfl/flCD4cre+, Kmt2d-/-) compared to wild type (Kmt2dfl/flCD4cre-, Kmt2dWT) mice. Second, we observed significant increase in death of nave CD8+ T cells upon stimulation in vitro. Third, we found reduced H3K4me1 amount in promoters and increased expressions of apoptosis-related genes in activated nave CD8+ T cells in the absence of Kmt2d. Finally, we confirmed increased activation-induced death of antigen specific nave CD8+ T cells in Kmt2d mice in vivo upon Listeria monocytogen infection. These findings reveal that Kmt2d regulates activation induced nave CD8+ T cell survival via modulating H3K4me1 levels in apoptosis-related gene expressions. Ageing has a profound detrimental impact on almost all living organisms. Immune systems play a particularly important role in protection against external challenges (pathogens) and internal insults (cancer) but their protective capacity commonly wanes with advancing age. With the rapid increase in the numbers of older people around the world, research in the field of immunity and ageing is becoming increasingly important. This realization, together with recent and ongoing technical advances in analytical capabilities, is facilitating rapid progress towards a better understanding of immunity and ageing and the resulting anticipated improved application of this knowledge to medical treatments in the years ahead.